bootctl: refer to kernel-install and add example (#22415)

pages/linux/bootctl.md

@@ -1,6 +1,7 @@
 # bootctl
 
 > Control EFI firmware boot settings and manage boot loader.
+> See also: `kernel-install`.
 > More information: <https://www.freedesktop.org/software/systemd/man/latest/bootctl.html>.
 
 - Show information about the system firmware and the bootloaders:
@@ -11,6 +12,10 @@
 
 `bootctl list`
 
+- Set a boot entry as the default to boot to:
+
+`sudo bootctl set-default {{entry_id}}`
+
 - Set a flag to boot into the system firmware on the next boot (similar to `sudo systemctl reboot --firmware-setup`):
 
 `sudo bootctl reboot-to-firmware true`